1. Technical Field
The present invention relates in general to a system and method for automatic voltage calibration. More particularly, the present invention relates to a system and method for automatically calibrating a reference voltage or a voltage regulator based upon a target voltage.
2. Description of the Related Art
Voltage reference circuitry is an important element for microprocessors, DSPs, and analog-to-digital conversion applications. When a device's voltage is scaled down to a low voltage level, a challenge found is achieving an accurate low voltage reference using traditional bandgap reference (BGR) topologies. Even though an adequate low voltage reference that is independent of temperature and supply voltage variations may be achieved using a BGR topology, a “process” variation component has become a major part of voltage reference err among reverence voltage PVT (process/voltage/temperature) variations. Factors that contribute to process variation are MOSFET mis-matching, which includes “no-zero” OPAMP offset voltage and current mirror mismatching, resistor variation, and diode variation.
Existing art may provide ways to reduce voltage reference err caused by process variation, such as using a centroid layout technique to reduce the local mismatch between passive and active components. However, a challenge found with this technique is that is does not help global mismatches caused by cross-wafer process variation. Another approach to reduce voltage reference err caused by process variation is to use an “auto-zero” technique to eliminate OPAMP offset voltage. However, a challenge found with this technique is that the technique is difficult to implement and, furthermore, the reference voltage is unavailable during sampling periods. Another approach to reduce process variation voltage reference errs is to use manufacturing “trimming.” A challenge found with this technique, however, is that it may increase manufacturing costs if the technique is not an automated process.
Similarly, voltage regulators use a reference voltage for regulating power that is provided to applications such as cell-phones, analog-to-digital converters, and on-chip power management of DSPs and microprocessors. A conventional voltage regulator includes a high gain err amplifier, a series power transistor, and a feedback network. The accuracy of such a voltage regulator is largely dependent upon the reference voltage. With today's processes scaling down to sub 90 nm regions, and supply voltages scaling down to below 1V, a challenge found is providing an accurate reference voltage at these levels.
Traditionally, voltage trimming has been used to obtain the desired temperature independent characteristics of a voltage regulator's reference voltage. However, as discussed above, process variation has become a major contributor of voltage reference err. Due to process variations, the reference voltage may have unacceptable three-sigma variations in a device. In principle, traditional voltage trimming may be used to solve process variation problems. However, a challenge found with this approach is that the traditional trimming method is a high cost and time consuming process and thus, is typically avoided in large volume, low cost applications.
What is needed, therefore, is a system and method to calibrate a reference voltage in a cost effective manner for use in production environments.